We can’t win the war in Afghanistan, so what do we do? We’ll train the Afghans to do it for us, then claim victory and head for the exits.

But what happens if we can’t train the Afghans?

We’re about to find out. It’s difficult to overstate just how calamitous the decision, announced Tuesday, to suspend most joint combat patrols between Afghan soldiers and their American and NATO mentors is. Preparing the Afghan Army and police to fight without us is the foundation of the Obama Administration’s strategy to withdraw most American forces—and have them stop fighting entirely—by the end of 2014. It’s our ticket home. As I outlined in a piece earlier this year, President Obama’s strategy amounts to an enormous gamble, and one that hasn’t, so far, shown a lot of promise. That makes this latest move all the more disturbing. We’re running out of time.

According to American military officers, the order suspends joint patrolling at the battalion-level and below without approval of a general. An American battalion is made up of about eight hundred soldiers; an Afghan battalion is about half that size. The overwhelming majority of foot patrols—and the overwhelming majority of the fighting with the Taliban—take place at this level. The order effectively means that, for now, Afghan soldiers and police operating in the field are largely on their own.

The decision, announced earlier this week, was prompted by the extraordinary rise in so-called green-on-blue attacks—the killing of Americans by Afghan soldiers. So far this year, more than fifty American and NATO soldiers have been killed by Afghan soldiers or recruits—a sixth of the three hundred and forty who have died this year. The most recent spate of green-on-blue deaths—four Americans on Sunday and two British on Saturday—coincided with a wave of anti-American riots around the world that followed the online distribution of a crude video depicting the Prophet Muhammad as a lecher and a fool.

In some ways, it would be comforting if the Afghans who were doing these killings were Taliban agents who’d slipped inside American training camps. There is some truth to this notion, but not much. When I was in Afghanistan this spring, a senior Afghan defense official told me that he and his fellow officials had little knowledge of the loyalties—or even the nationalities—of many of the new recruits. Many, he said, were presumed to have been sent by Pakistani intelligence officials from across the border. An American official told me that “several hundred” Afghan recruits, including some officers, had been identified as loyal either to the Taliban or to the Pakistanis.

As bad as that sounds, though, the reality is much worse. By the Americans’ own accounting, only ten per cent of the green-on-blue attacks have been carried out by Taliban infiltrators. The overwhelming majority of green-on-blue attacks are coming from ordinary Afghans signing up for the military. The very people we are trying to help fight the Taliban are turning their guns on us.

You can imagine the level of anxiety among American and European trainers who are sharing bases with Afghan recruits. Recent measures announced by American commanders to protect their soldiers and marines haven’t been enough. And how could they be? It’s the nature of the American mission to walk “shoulder-to-shoulder” with Afghan soldiers, who are often just as well armed as the Americans are. All you have to do is turn your gun on the guy walking next to you.

Which leads us back to the decision to ban joint patrols. When I was in Afghanistan in April and May, it was clear that the entire thrust of the American-led mission was to train Afghan forces as quickly as possible, so that we could stay on our departure schedule. The principal means to that end, I was told repeatedly by senior commanders, was carrying out operations together. (I heard the phrase “shoulder to shoulder” so often I started repeating it in my sleep.) The reason for this was simple: Afghan soldiers typically perform much better when American soldiers are with them. The Afghans fight better, feel better, and abuse each other, and Afghan civilians, less. Until this week’s decision, the commanders told me, the overwhelming majority of military operations were carried out by Afghans and Americans on joint operations. As of today, this is no longer true.

I suppose it’s possible that the decision to ban joint patrols will be lifted in a week or so, and that everything will return to normal. Possible—but not likely. The Americans have been in Afghanistan for eleven years. We can’t remain much longer because we’ve overstayed our welcome, even among our friends. And we can’t leave, either, because leaving could lead to a Taliban takeover or a very bloody civil war—and now we can barely train the local troops to take over for us.

We can’t leave and we can’t stay: that’s the very definition of a quagmire.